Communication receiver with hybrid equalizer

ABSTRACT

Wireless communication receiver with hybrid equalizer and RAKE receiver. The receiver compares performance of the system for RAKE only and RAKE in combination with equalizer estimates. The receiver enables or disables the equalizer accordingly.

CLAIM OF PRIORITY UNDER 35 U.S.C. §119

[0001] The present Application for Patent claims priority to ProvisionalApplication No. 60/475,250 entitled “COMMUNICATION RECEIVER WITH HYBRIDEQUALIZER” filed Jun. 2, 2003, and assigned to the assignee hereof.

BACKGROUND

[0002] 1. Field

[0003] The present invention relates generally to equalization incommunications systems, and more specifically, to a universal receiverincorporating a RAKE receiver and a hybrid equalizer.

[0004] 2. Background

[0005] Communications systems are used for transmission of informationfrom one device to another. Prior to transmission, information isencoded into a format suitable for transmission over a communicationchannel. The transmitted signal is distorted as it travels through thecommunication channel; the signal also experiences degradation fromnoise and interference picked up during transmission. An example ofinterference commonly encountered in band-limited channels is calledinter-symbol interference (ISI). ISI occurs as a result of the spreadingof a transmitted symbol pulse due to the dispersive nature of thechannel, which results in an overlap of adjacent symbol pulses.

[0006] The received signal is decoded and translated into the originalpre-encoded form. Both the transmitter and receiver are designed tominimize the effects of channel imperfections and interference. For thepurposes of this disclosure, interference or distortion due to channelimperfections, or any combination thereof will be referred to generallyas noise.

[0007] Various receiver designs may be implemented to compensate fornoise caused by the transmitter and the channel. By way of example, anequalizer is a common choice for dealing with ISI. An equalizer correctsfor distortions and generates an estimate of the transmitted symbol. Inthe wireless environment, equalizers are required to handle time-varyingchannel conditions. Ideally, the response of the equalizer adjusts tochanges in channel characteristics.

[0008] Equalizers are generally complex, tending to increase the powerconsumption of a communication device. A need exists, therefore, for anequalizer design that reduces power consumption. Further, there is aneed for controlling an equalizer so as operate the equalizer duringsuch channel conditions as result in optimum performance of theequalizer. Still further there is a need to implement an equalizer inparallel with a RAKE receiver, wherein the equalizer only operatesduring specified operating conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a portion of a rake receiver in a communication system;

[0010]FIG. 2A is model of a communication system;

[0011]FIG. 2B is a model of a transmission portion of a communicationsystem, including modulation and analog receiver processing;

[0012]FIG. 3 is receive data processor in a mobile station;

[0013]FIG. 4 is a receiver supporting data communications;

[0014]FIG. 5 is a state diagram illustrating operation of a receiveremploying a RAKE and hybrid equalizer; and

[0015]FIG. 6 is a state diagram illustrating operation of a receiverincorporating multiple operational states.

DETAILED DESCRIPTION

[0016] The word “exemplary” is used herein to mean “serving as anexample, instance, or illustration.” Any embodiment described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments.

[0017] Communications systems are used for transmission of informationfrom one device to another. Before transmission, information is encodedinto a format suitable for transmission over a communication channel.The communication channel may be a transmission line or free spacebetween the transmitter and the receiver. As the signal propagatesthrough the channel, the transmitted signal is distorted byimperfections in the channel. Furthermore, the signal experiencesdegradation from noise and interference picked up during transmission.An example of interference commonly encountered in band-limited channelsis called inter-symbol interference (ISI). ISI occurs as a result of thespreading of a transmitted symbol pulse due to the dispersive nature ofthe channel, which results in an overlap of adjacent symbol pulses. Atthe receiver, the signal is processed and translated into the originalpre-encoded form. Both the transmitter and receiver are designed tominimize the effects of channel imperfections and interference. For thepurposes of this disclosure, interference or distortion due to channelimperfections, or any combination thereof will be referred to generallyas noise.

[0018] Various receiver designs may be implemented to compensate fornoise caused by the transmitter and the channel. In one design, a RAKEreceiver is implemented. In another design an equalizer is used. Instill another design a RAKE receiver and an equalizer are bothimplemented.

[0019] RAKE Configuration

[0020] A communication system may employ a RAKE receiver to process amodulated signal transmitted on the forward link or reverse link. TheRAKE receiver typically includes a searcher element and a number offinger processors. The searcher element searches for strong instances ofthe received signal (or multipaths). The finger processors are assignedto process the strongest multipaths to generate demodulated symbols forthose multipaths. The RAKE receiver then combines the demodulatedsymbols from all assigned finger processors to generate recoveredsymbols that are estimates of the transmitted data. The RAKE receiverefficiently combines energy received via multiple signal paths.

[0021] The RAKE receiver provides an acceptable level of performance forCDMA systems operated at low signal-to-noise ratio (S/N). For CDMAsystems designed to transmit data at high data rates, such as the HDRsystem, higher S/N is required. To achieve the higher S/N, thecomponents that make up the noise term N need to be reduced. The noiseterm includes thermal noise (No), interference (Io) due to transmissionsby other transmitting sources and transmissions for other users, andinter-symbol interference (ISI) that can come from multipath anddistortion in the transmission channel. For CDMA systems designed tooperate at low S/N, the ISI component is typically negligible comparedto other noise components. However, for CDMA systems designed to operateat higher S/N, the other noise components (e.g., interference from othertransmission sources) are typically reduced and ISI becomes anon-negligible noise component that may have a large impact on theoverall S/N.

[0022] As noted above, the RAKE receiver provides acceptable performancewhen the S/N of the received signal is low. The RAKE receiver can beused to combine energy from various multipaths, but generally does notremove the effects of ISI (e.g., from multipath and channel distortion).Thus, the RAKE receiver may not be capable of achieving the higher S/Nrequired by systems operating at higher data rates.

[0023]FIG. 1 is a block diagram of an embodiment of rake receiver 100.Due to multipath and other phenomena, a transmitted signal may reach areceiver unit via multiple signal paths. For improved performance, therake receiver is designed with the capability to process multiple (andstrongest) instances of the received signal (or multipaths). For aconventional rake receiver design, a number of finger processors 110 areprovided to process a number of multipaths. Each finger processor 110comprises a finger of the rake receiver and can be assigned to process aparticular multipath.

[0024] In a spread-spectrum communication system, such as a CodeDivision Multiple Access (CDMA) system, the received In-phase (I_(IN))and Quadrature (Q_(IN)) samples from a particular pre-processor (notshown) are provided to a number of finger processors 110 a through 110l. Within each assigned finger processor 110, the received I_(IN) andQ_(IN) samples are provided to a PN despreader 120, which also receivesa complex PN sequence, PNI and PNQ. The complex PN sequence is generatedin accordance with the particular design of the CDMA system beingimplemented and, for the HDR system, is generated by multiplying theshort IPN and QPN sequences with the long PN sequence by multipliers 138a and 138 b. The short IPN and QPN sequences are used to spread the dataat the transmitting base station, and the long PN sequence is assignedto the recipient receiver unit and used to scramble the data. The PNIand PNQ sequences are generated with a time offset corresponding to theparticular multipath being processed by that finger processor.

[0025] PN despreader 120 performs a complex multiply of the complexI_(IN) and Q_(IN) samples with the complex PN sequence and providescomplex despread I_(DES) and Q_(DES) samples to decover elements 122 and132. Decover element 122 decovers the despread samples with one or morechannelization codes (e.g., Walsh codes) that were used to cover thedata and generates complex decovered samples. The decovered samples arethen provided to a symbol accumulator 124 that accumulates the samplesover the length of the channelization codes to generate decoveredsymbols. The decovered symbols are then provided to a pilot demodulator126.

[0026] For a High Rate Packet Date (HRPD) system, such as specified byIS-856, a pilot reference is transmitted during a portion of the forwardlink transmission. Thus, decover element 132 decovers the despreadsamples with the particular channelization code (e.g., a Walsh code 0for the HDR system) that was used to cover the pilot reference at thebase station. The decovered pilot samples are then provided to anaccumulator 134 and accumulated over a particular time interval togenerate pilot symbols. The accumulation time interval can be theduration of the pilot channelization code, an entire pilot referenceperiod, or some other time interval. The pilot symbols are then providedto a pilot filter 136 and used to generate pilot estimates that areprovided to pilot demodulator 126. The pilot estimates are estimated orpredicted pilot symbols for the time period when data is present.

[0027] Pilot demodulator 126 performs coherent demodulation of thedecovered symbols from symbol accumulator 124 with the pilot estimatesfrom pilot filter 136 and provides demodulated symbols to a symbolcombiner 140. Coherent demodulation can be achieved by performing a dotproduct and a cross product of the decovered symbols with the pilotestimates. The dot and cross products effectively perform a phasedemodulation of the data and further scale the resultant output by therelative strength of the recovered pilot. The scaling with the pilotseffectively weighs the contributions from different multipaths inaccordance with the quality of the multipaths for efficient combining.The dot and cross products thus perform the dual role of phaseprojection and signal weighting that are characteristics of a coherentrake receiver.

[0028] Symbol combiner 140 receives and coherently combines thedemodulated symbols from all assigned finger processors 110 to providerecovered symbols for a particular received signal being processed bythe rake receiver. The recovered symbols for all received signals maythen be combined, as described below, to generate overall recoveredsymbols that are then provided to the subsequent processing element.

[0029] Searcher element 112 can be designed to include a PN despreader,a PN generator, and a signal quality measurement element. The PNgenerator generates the complex PN sequence at various time offsets,possibly as directed by a controller (not shown), which are used in thesearch for the strongest multipaths. For each time offset to be search,the PN despreader receives and despreads the I_(IN) and Q_(IN) sampleswith the complex PN sequence at the particular time offset to providedespread samples. The signal quality of the despread samples is thenestimated. This can be achieved by computing the energy of each despreadsample (i.e., I_(DES) ²+Q_(DES) ²) and accumulating the energy over aparticular time period (e.g., the pilot reference period). Searcherelement performs the search at numerous time offsets, and the multipathshaving the highest signal quality measurements are selected. Theavailable finger processors 110 are then assigned to process thesemultipaths.

[0030] The design and operation of a rake receiver for a CDMA system isdescribed in further detail in U.S. Pat. No. 5,764,687, entitled “MOBILEDEMODULATOR ARCHITECTURE FOR A SPREAD SPECTRUM MULTIPLE ACCESSCOMMUNICATION SYSTEM,” and U.S. Pat. No. 5,490,165, entitled“DEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVINGMULTIPLE SIGNALS,” both assigned to the assignee of the presentinvention.

[0031] In one embodiment, a number of forward link signals are receivedby k antennas and processed to generate sample streams x₁ (n) throughX_(K) (n). Thus, a number of rake receivers may be provided to processthe k sample streams. A combiner may then be used to combine therecovered symbols from all received signals being processed.Alternatively, one or more rake receivers can be time divisionmultiplexed to process the K sample streams. In such a Time DivisionMultiplex (TDM) rake receiver architecture, the samples from the kstreams may be temporarily stored to a buffer and later retrieved andprocessed by the rake receiver.

[0032] For each received signal, rake receiver 100 may be operated toprocess up to L multipaths, where 1 represents the number of availablefinger processors 110. Each of the 1 multipaths corresponds to aparticular time offset identified with the assistance of searcherelement 112. A controller or searcher element 112 may be designed tomaintain a list of the magnitude of the strongest multipath (α_(ji)) andcorresponding time offset (τ_(i)) for each of the k received signalsbeing processed.

[0033] In the combination receiver configuration having a RAKE and anequalizer, these magnitudes and time offsets can be used to initializethe coefficients and scaling factors of equalizer, as described above.In one implementation, the magnitude of each multipath of interest canbe computed as the square root of the accumulated energy value dividedby the number of samples (N) used in the accumulation.

[0034] Equalizer Configuration

[0035] An equalizer is a common choice for dealing with ISI. Anequalizer may be implemented with a transversal filter, i.e. a delayline with T-second taps (where T is the symbol duration). The contentsof the taps are amplified and summed to generate an estimate of thetransmitted symbol. The tap coefficients are adjusted to reduceinterference from symbols that are adjacent in time to the desiredsymbols. Commonly, an adaptive equalization technique is employedwhereby the tap coefficients are continually and automatically adjusted.The adaptive equalizer uses a prescribed algorithm, such as Least MeanSquare (LMS) or Recursive Least Squares (RLS), to determine the tapcoefficients. The symbol estimate is coupled to a decision-making devicesuch as a decoder or a symbol slicer.

[0036] The ability of a receiver to detect a signal in the presence ofnoise is based on the ratio of the received signal power and the noisepower. This ratio is commonly known as the signal-to-noise power ratio(SNR), or the carrier-to-interference ratio (C/I). Industry usage ofthese terms, or similar terms, is often interchangeable, however, themeaning is the same. Accordingly, any reference to C/I herein will beunderstood by those skilled in the art to encompass the broad concept ofmeasuring the effects of noise at various points in the communicationssystem.

[0037] Typically, the C/I may be determined in the receiver byevaluating symbol estimates of a known transmitted symbol sequence. Thismay be accomplished in the receiver by computing the C/I for thetransmitted pilot signal. Since the pilot signal is known, the receivermay compute the C/I based on the symbol estimates from the equalizer.The resultant C/I computation may be used for a number of purposes. Incommunications systems employing a variable rate data request scheme,the receiver may communicate to the transmitter the maximum data rate itmay support based on the C/I. Furthermore, if the receiver includes aturbo decoder, then depending on the transmitted constellation, the LogLikelihood Ratio (LLR) computation needs an accurate estimate of theC/I.

[0038] Equalizers in wireless communication systems are designed toadjust to time varying channel conditions. As the channelcharacteristics change, the equalizer adjusts its response accordingly.Such changes may include variations in the propagation medium or therelative motion of the transmitter and receiver, as well as otherconditions. As discussed hereinabove, adaptive filtering algorithms areoften used to modify the equalizer tap coefficients. Equalizers thatemploy adaptive algorithms are generally referred to as adaptiveequalizers. Adaptive algorithms share a common property: adaptationspeed decreases as the number of equalizer taps increases. Slowadaptation impacts the tracking behavior of adaptive equalizers. A“long” equalizer, i.e., an equalizer having a large number of taps, isdesirable as long equalizers more accurately invert channel distortionsresulting in good steady state performance. Long equalizers, however,react more slowly to channel variations leading to poor transientbehavior, i.e., poor performance when the channel is rapidly varying. Anoptimum number of taps balances such considerations and compromisesbetween good steady-state performance and good transient performance.

[0039] In practice, determining the optimum number of taps is difficultas the optima depends on a variety of conditions and goals, includingbut not limited to, the instantaneous response of the channel, and therate of variation of the channel. So it is difficult to determine, apriori, the optimum number of taps if the equalizer is to be used on avariety of channels, in a variety of time-varying conditions.

[0040]FIG. 2A illustrates a portion of the components of a communicationsystem 200. Other blocks and modules may be incorporated into acommunication system in addition to those blocks illustrated. Bitsproduced by a source (not shown) are framed, encoded, and then mapped tosymbols in a signaling constellation. The sequence of binary digitsprovided by the source is referred to as the information sequence. Theinformation sequence is encoded by encoder 202 which outputs a bitsequence. The output of encoder 202 is provided to mapping unit 204,which serves as the interface to the communication channel. The mappingunit 104 maps the encoder output sequence into symbols γ(n) in a complexvalued signaling constellation. Further transmit processing, includingmodulation blocks, as well as the communication channel and analogreceiver processing, are modeled by section 220.

[0041]FIG. 2B illustrates some of the details included within section220 of FIG. 2A. As illustrated in FIG. 2B, the complex symbols γ(n) aremodulated onto an analog signal pulse, and the resulting complexbaseband waveform is sinuosoidally modulated onto the in-phase andquadrature-phase branches of a carrier signal. The resulting analogsignal is transmitted by an RF antenna (not shown) over a communicationchannel. A variety of modulation schemes may be implemented in thismanner, such as M-ary Phase Shift Keying (M-PSK), 2^(M)-ary QuadratureAmplitude Modulation (2^(M) QAM), etc.

[0042] Each modulation scheme has an associated “signalingconstellation” that maps one or more bits to a unique complex symbol.For example, in 4-PSK modulation, two encoded bits are mapped into oneof four possible complex values {1,i ,-1,-i}. Hence each complex symbolγ(n) may take on four possible values. In general for M-PSK, log₂Mencoded bits are mapped to one of M possible complex values lying on thecomplex unit circle.

[0043] Continuing with FIG. 2B, at the receiver, the analog waveform isdown-converted, filtered and sampled, such as at a suitable multiple ofthe Nyquist rate. The resulting samples are processed by the equalizer210, which corrects for signal distortions and other noise andinterference introduced by the channel, as modeled by section 220. Theequalizer 210 outputs estimates of the transmitted symbols γ(n). Thesymbol estimates are then processed by a decoder to determine theoriginal information bits, i.e., the source bits that are the input toencoder 202.

[0044] The combination of a pulse-filter, an I-Q modulator, the channel,and an analog processor in the receiver's front-end, illustrated in FIG.2A and FIG. 2B, is modeled by a linear filter 206 having an impulseresponse {h_(k)} and a z-transform H(z), wherein the interference andnoise introduced by the channel are modeled as Additive White GaussianNoise (AWGN).

[0045]FIG. 2B details processing section 220 as including a front endprocessing unit 222 coupled to baseband filters 226 and 228 forprocessing the In-phase (I) and Quadrature (Q) components, respectively.Each baseband filter 226, 228 is then coupled to a multiplier formultiplication with a respective carrier. The resultant waveforms arethen summed at summing node 234 and transmitted over the communicationchannel to the receiver. At the receiver, an analog pre-processing unit242 receives the transmitted signal, which is processed and passed to amatched filter 244. The output of the matched filter 244 is thenprovided to an Analog/Digital (A/D) converter 246. Note that othermodules may be implemented according to design and operational criteria.The components and elements of FIG. 2A and 2B are provided for anunderstanding of the following discussion and are not intended to be acomplete description of a communication system.

[0046] RAKE and Equalizer Combination

[0047] In another design, a RAKE receiver is operated in parallel withan equalizer. Such a design is detailed in “METHOD AND APPARATUS FORPROCESSING A MODULATED SIGNAL USING AN EQUALIZER AND A RAKE RECEIVER,”by John Smee et al., having Application No. 09/624,319, filed Jul. 24,2000. A selection is made between the RAKE receiver and the equalizer todetermine the best estimate of the received signal. For example, theselection may correspond to the lowest Mean Square Error (MSE) between atransmitted pilot signal and the estimate, or the highest Signal toInterference and Noise Ratio (SINR) at each output, or some othercriteria. The performance measure or estimate provides a means forcomparing the RAKE and the equalizer. The selected receiverconfiguration is then used for processing the received data signal.

[0048] A receiver is termed “universal” if its performance is optimumover the “universe” of possible channel conditions and rates of channelvariation. The receiver with a RAKE and an equalizer is “universal” ifthe receiver configuration selected on the basis of the MSE estimate orC/I estimate is, in fact, the best configuration among the twoconfigurations. Thus accurate MSE estimates or C/I estimates arenecessary to make a receiver “universal.”

[0049]FIG. 3 is a block diagram of receive data processor 310 within amobile station 300 in accordance with an embodiment of the invention. Inthis embodiment, receive data processor 310 includes two signalprocessing paths that can be operated in parallel to provide improvedperformance, especially at higher data rates. The first signalprocessing path includes an equalizer 312 coupled to a post processor314 and the second signal processing path includes a RAKE 316.

[0050] Within receive data processor 310, the streams of samples frompre-processors (not shown) are provided to each of equalizer 312 andRAKE 316. Each stream of samples is generated from a respective receivedsignal. Equalizer 312 performs equalization on the received streams ofsamples and provides symbol estimates to post processor 314. Dependingon the processing performed at transmission, post processor 314 mayfurther process the symbol estimates to provide recovered symbols. Inparticular, if PN spreading and covering are performed at thetransmitter unit, post processor 314 may be configured to performdespreading with a complex PN sequence and decovering with one or morechannelization codes. Phase rotation (which is achieved via pilotdemodulation for a rake receiver) is implicitly achieved by equalizer312 after the filter coefficients have been adopted.

[0051] RAKE 316 may be configured to process one or more multipaths ofeach received signal to provide recovered symbols for that receivedsignal. For each stream of samples, RAKE 316 may be configured toperform PN despreading, decovering, and coherent demodulation for anumber of multipaths. RAKE 316 then combines demodulated symbols for allmultipaths of a received signal to generate recovered symbols for thatreceived signal. RAKE 316 may further combine the recovered symbols forall received signals to provide the overall recovered symbols that areprovided from the rake receiver.

[0052] The recovered symbols from post processor 314 and RAKE 316 may beprovided to a switch (SW) 320 that selects the recovered symbols fromeither post processor 314 or RAKE 316 to provide to a de-interleaver322. The selected recovered symbols are then reordered by de-interleaver322 and subsequently decoded by a decoder 324. A controller 318 couplesto, and manages the operation of equalizer 312, post processor 314, rakereceiver 316, and switch 320.

[0053] In accordance with the invention, equalizer 312 may be used toprovide equalization of the received signals to reduce the amount of ISIin the received signals. Each received signal is distorted by thecharacteristics of the transmitter unit, the transmission channel, andthe receiver unit. Equalizer 312 may be operated to equalize the overallresponse for each received signal, thus reducing the amount of ISI. Thelower ISI improves S/N and may support higher data rates.

[0054] Continuing with FIG. 3, receive data processor 310 includes twosignal processing paths that can be operated to process the receivedsignals. The first signal processing path includes equalizer 310 andpost processor 314, and the second signal processing path includes RAKE316. In an embodiment, the two signal processing paths can be operatedin parallel (e.g., during the adaptation period) and a signal qualityestimate can be computed for each of the signal processing paths. Thesignal processing path that provides the better signal quality can thenbe selected to process the received signals.

[0055] For a conventional RAKE, the received signal quality can beestimated by computing the signal-to-noise (SIN) ratio. For CDMA systemsthat transmit TDM pilot reference, the S/N can be computed during thepilot reference period when the received signal is known. A signalquality estimate can be generated for each assigned finger processor.The estimates for all assigned finger processors can then be weightedand combined to generate an overall S/N, which can be computed as:$\begin{matrix}{{S/N_{RAKE}} = \frac{\left( {\sum\limits_{i = 1}^{K}\quad {\beta_{i} \cdot \sqrt{E\quad s_{i}}}} \right)^{2}}{\sum\limits_{i = 1}^{K}\quad {{\beta_{i}^{2} \cdot N}\quad t_{i}}}} & {{Eq}.\quad (1)}\end{matrix}$

[0056] where β is the weighting factors used by the rake receiver tocombine the demodulated symbols from the assigned finger processors toprovide the recovered symbols that are improved estimates of thetransmitted data, Es is the energy-per-symbol for the desired signal(e.g., the pilot) and Nt is the total noise on the received signal beingprocessed by the finger processor. Nt typically includes thermal noise,interference from other transmitting base stations, interference fromother multipaths from the same base station, and other components. Theenergy-per-symbol can be computed as: $\begin{matrix}{{{E\quad s} = {\frac{1}{N_{SYM}}{\sum\limits_{i = 1}^{N_{SYM}}\quad \left( {{P_{I}^{2}(i)} + {P_{Q}^{2}(i)}} \right)}}},} & {{Eq}.\quad (2)}\end{matrix}$

[0057] where P_(I) and P_(Q) are the in phase and quadrature filteredpilot symbols and N_(SYM) is the number of symbols over which the energyis accumulated to provide the Es value. The filtered pilot symbols canbe generated by accumulating the despread samples over the length of thechannelization code used to cover the pilot reference. The total noisecan be estimated as the energy of the variations in the energy of thedesired signal, which can be computed as: $\begin{matrix}{{N\quad t} = {\frac{1}{N_{SYM} - 1}{\left\{ {{\sum\limits_{i = 1}^{N_{SYM}}\quad \left( {{P_{I}^{2}(i)} + {P_{Q}^{2}(i)}} \right)} - {\frac{1}{N_{SYM}}\left( {\sum\limits_{i = 1}^{N_{SYM}}{P_{I}(i)}} \right)^{2}} - {\frac{1}{N_{SYM}}\left( {\sum\limits_{i = 1}^{N_{SYM}}{P_{Q}(i)}} \right)^{2}}} \right\}.}}} & {{Eq}.\quad (3)}\end{matrix}$

[0058] The measurement of the received signal quality is described infurther detail in U.S. Pat. No. 5,903,554, entitled “METHOD ANDAPPARATUS FOR MEASURING LINK QUALITY IN A SPREAD SPRECTRUM COMMUNICATIONSYSTEM,” and U.S. Pat. No. 5,799,005, entitled “SYSTEM AND METHOD FORDETERMINING RECEIVED PILOT POWER AND PATH LOSS IN A CDMA COMMUNICATIONSYSTEM,” both assigned to the assignee of the present invention.

[0059] For the signal processing path that includes equalizer 312, thesignal quality may be estimated using various criteria, including a meansquare average (MSE). Again, for CDMA systems that transmit TDM pilotreference, the MSE can be estimated during the pilot reference period,and can be computed as: $\begin{matrix}{{{MSE} = {\frac{1}{N_{SAM}}{\sum\limits_{n = 1}^{N_{SAM}}{{{y(n)} - {\hat{y}(n)}}}^{2}}}},} & {{Eq}.\quad (4)}\end{matrix}$

[0060] where N_(SAM) is the number of samples over which the error isaccumulated to provide the MSE. Typically, the mean square error isaveraged over a number of samples, and over one or more pilotreferences, to obtain a desired level of confidence in the measurement.The mean square error can then be translated to an equivalentsignal-to-noise ratio, which can be expressed as: $\begin{matrix}\begin{matrix}{{S/N_{EQ}} = {\frac{1}{MSE} - {1\quad {linear}}}} \\{= {10\quad {\log \left( {\frac{1}{MSE} - 1} \right)}\quad {dB}}}\end{matrix} & {{Eq}.\quad (5)}\end{matrix}$

[0061] The S/N_(EQ) for the signal processing path with equalizer 312can be compared with the S/N_(RAKE) for the signal processing path withRAKE 316. The signal processing path that provides the better S/N canthen be selected to process the received signals.

[0062] Alternatively, the MSE can be computed for the signal processingpath with RAKE 316 and compared against the MSE computed for the signalprocessing path with equalizer 312. The signal processing path with thebetter MSE may then be selected.

[0063] For a HRPD system, the S/N is estimated at a remote terminal andused to determine a maximum data rate that can be received by the remoteterminal for the operating conditions. The maximum data rate is thentransmitted back to the base station for which the S/N is estimated.Thereafter, that base station transmits to the remote terminal at a datarate up to identified maximum data rate.

[0064] With the present invention, the data rate for a data transmissioncan be estimated using various methods. In one method, the S/N can beestimated for the RAKE or for the equalizer based on the computed MSE.The best S/N from all signal processing paths can then be used todetermine a maximum data rate that can be supported. Alternatively, theMSE can be used to directly determine the maximum data rate. The bestS/N, MSE, or maximum data rate can be sent to the base station.

[0065] Under certain operating conditions, the signal processing pathwith the equalizer can provide better performance than the one with therake receiver. For example, the signal processing path with theequalizer typically performs better when the S/N is high and forchannels with ISI. The RAKE can be used to process multipaths, whichalso cause ISI. In fact, the RAKE can be viewed as a filter with L taps(where L corresponds to the number of finger processors), with each tapcorresponding to a time delay that can be adjusted. However, the RAKE isnot as effective at reducing ISI due to frequency distortion in thereceived signals.

[0066] The equalizer may more effectively reduce ISI due to frequencydistortion. This is achieved by providing a response that isapproximately the inverse of the frequency distortion while attemptingto minimize the overall noise, which includes the ISI. The equalizerthus “inverts” the channel and also attempts to smooth out the effect ofmultipath. In fact, each filter 410, when the coefficients areinitialized to {0, . . . , 0, 1, 0, . . . ,0}, is equivalent to onefinger processor. Subsequently, as the zero-valued coefficients areadapted, the filter frequency response is altered to equalize thechannel distortion. Thus, the equalizer may be used to effectively dealwith both multipath-induced ISI and channel-induced ISI.

[0067] For simplicity, many of the aspects and embodiments of theinvention have been described for a spread spectrum communicationsystem. However, many of the principles of the invention describedherein can be applied to non- spread spectrum communication systems, andcommunication systems capable of selectively performing direct sequencespreading, such as the HRPD system.

[0068] RAKE and Hybrid Equalizer Configuration

[0069] According to one embodiment, the equalizer 312 may be a hybridequalizer, wherein equalizer 312 is turned on when operating conditions,including but not limited to channel conditions, encourage the user ofan equalizer. In other words, when the equalizer 312 is expected toperform as well as or better than the RAKE 316, then the equalizer 312is turned on. Else, the equalizer is not operated. In this way, thesystem experiences power savings during those times when the equalizeris expected to perform worse than the RAKE 316. Such an equalizer isreferred to as a “hybrid” equalizer, as the equalizer is responsive tooperating conditions.

[0070] A hybrid equalizer and RAKE receiver architecture operates bycomparing an operating condition metric, such as the potentialdemodulation SINR outputs of a RAKE and equalizer, and then selectingthe mode that achieves the best performance. Modes may include, but arenot limited to, RAKE only mode, and RAKE and equalizer mode. Oneembodiment includes a test mode to periodically run the equalizer andselect between the RAKE only and RAKE and equalizer modes. The hybridequalizer will typically have better performance for higher geometry andslow fading conditions. In such conditions, the equalizer offersperformance gains as compared to a conventional RAKE only design. In thesimplest implementation, however, the cost of running both methods maybe prohibitive, incurring increased power dissipation even for thoseconditions for which the equalizer offers no gain over the RAKE.

[0071] Ideally the equalizer only operates when gains in performance maybe realized. The hybrid equalizer provides power reduction by employinga decision algorithm based on short temporal operating conditions, suchas correlation statistics and/or receiver SINR. The hybrid equalizer isoperated only when the channel conditions are likely to yieldperformance gains.

[0072] As the equalizer relies on slow fading channel conditions, oneembodiment estimates fading dynamics by estimating the inter-pilot burstcorrelation statistics. An equalizer typically yields gains for highergeometry (i.e., SINR), wherein another embodiment estimates SINR fromthe pilot burst. The two metrics may both be used in a decisionalgorithm. If the correlation metric is above a given threshold and theSINR is also above another threshold, then the equalizer is enabledotherwise the equalizer is disabled. This reduces the power dissipationby avoiding the use of the equalizer when no benefit is achieved.

[0073]FIG. 5 is a state diagram illustrating operation 500 of a receiveraccording to one embodiment implementing a RAKE and hybrid equalizer.Two modes are implemented: RAKE only mode 502; and RAKE and hybridequalizer mode 504.

[0074] Operation starts in the RAKE only mode. While in mode 502,operation is maintained in mode 502 until there is a change in operatingconditions sufficient to indicate performance increases would beachieved with addition of the equalizer. To determine if operatingconditions have changed sufficiently, such as a slowly varying channelcondition, a channel quality metric is evaluated. In the presentembodiment, the channel quality metric is the Signal to Interference andNoise Ratio (SINR) of the RAKE output (SINR_(RAKE)) is measured andcompared to a threshold value (T_(EQU)) for triggering the equalizer.Similarly, a correction metric (C_(RAKE)) is determined for the RAKE andcompared to a corresponding correction metric (C_(EQU)) for theequalizer. When SINR_(RAKE) is greater than T_(EQU), and C_(RAKE) isgreater than C_(EQU)) then operation transitions to a RAKE and equalizermode 504. In this way, when operating conditions encourage use of theequalizer, mode 504 is entered and the equalizer operation begins.

[0075] While in mode 504 the system continues to monitor the SINR of theRAKE output and the equalizer output (SINR_(EQU)). When SINR_(RAKE) isgreater than SINR_(EQU), operation transitions to mode 502. The use ofan equalizer is typically encouraged as the SINR increases, as SINR (asa function of current geometry of the system) indicates the channelcondition. For low SINR, the equalizer does not perform as well, andtherefore SINR acts as a good trigger for turning the equalizer on andoff. The trigger for entering the mode 504, i.e., enabling theequalizer, is effectively a two-part consideration. The first evaluationdetermines if the channel condition, e.g., SINR, is consistent withthose conditions for which equalizer operation improves performance. Thesecond evaluation determines the speed of the channel, or in otherwords, how quickly a mobile station is moving in the cellular network.According to one embodiment, the second evaluation determines thecross-correlation between pilot bursts. Cross-correlation measures thedegree to which two series are correlated. In this case, as thecorrelation of the signals increases, the delay between the two signalsdecreases. Similarly, the correlation decreases as the delay increases.Therefore, as the distance between the mobile station and the receiverincreases, or changes, there is a decrease in the correlation of signalsreceived. For a low cross-correlation, the equalizer is enabled in mode504, else the RAKE only mode 502 is maintained. The cross-correlationmay be measured on the pilot signal, or pilot burst, as this is a knownsignal providing confidence in the result.

[0076] As an example, consider the cross correlation metric as follows.Given a pilot symbol, P_(□), the correlation between successive pilotsymbols may be estimated as: $\begin{matrix}{C_{RAKE} = {\frac{\sum\limits_{k = 1}^{N_{SUM}}\quad {P_{\mu}P_{k + 1}^{*}}}{\sum\limits_{\mu = 1}^{N_{SUM}}\quad {P_{\mu}P_{\mu}^{*}}}}} & {{Eq}.\quad (6)}\end{matrix}$

[0077] by averaging over N_(SUM) pilot symbols. The correlation metricranges from 0 to 1. Correlation of 1 implies a strong correlation and islikely to yield good equalizer performance, as the channel is notchanging between successive pilot symbols. Alternate embodiments maydefine other parameters and metrics, which trigger an equalizer. Metricsmay be selected which provide an expected equalizer performance. Themetrics may be selected specific to the system design and performancegoals.

[0078]FIG. 6 is a state diagram of operation 520 for an alternateembodiment having three modes of operation. In a RAKE only mode 522 theRAKE is used while the equalizer is not operating. Periodically,measured by a sample period, which may be prespecified or adaptive,operation enters a test mode 524.

[0079] During the test mode 524, equalizer operation is enabled. Thetest mode 524 enables the equalizer to determine if the performance ofthe equalizer adds to the performance of the receiver. The results ofthe RAKE and equalizer are compared to evaluate performance of theequalizer. When SINR_(RAKE) is less than SINR_(EQU), operationtransitions to RAKE and equalizer mode 526, wherein both the RAKE andequalizer are enabled. In this situation, the equalizer shows capacityfor improving performance. If the results in test mode 524 indicate thatSINR_(RAKE) is greater than SINR_(EQU), operation transitions back tomode 522 for RAKE only. In this situation the equalizer does not improveperformance, and is not expected to provide an overall improvement inperformance under the current conditions. Note, a margin value (δ) maybe illustrated, wherein SINR_(RAKE) or SINR_(EQU) is biased according tosystem design and/or performance. The sample period may be designed as afunction of the time required to operate the equalizer, wherein thesample period is sufficient to allow data to traverse the filterelements of the equalizer.

[0080] While in mode 526, the system monitors the channel qualitymetric. When SINR_(RAKE) is greater than SNR_(EQU), operationtransitions to RAKE only mode 522. Note that transitions implement amargin value (δ) so as to avoid toggling between modes. In this way,transitions from mode 524 to mode 526 occur when the SINR of theestimate generated by the equalizer exceeds the SINR generated by theRAKE by more than a margin. Similarly, transitions from mode 524 to mode522 occur when the SINR of the estimate generated by the RAKE exceedsthe SINR generated by the equalizer by more than a margin. Additionally,transitions from mode 526 to mode 522 occur when the SINR of theestimate generated by the RAKE exceeds the SINR generated by theequalizer by more than a margin.

[0081] High Rate Packet Data Communication Systems

[0082] Throughout the following discussion a specific high data ratesystem is described for clarity. Alternate systems may be implementedthat provide transmission of information at high data rates. For CDMAcommunications systems designed to transmit at higher data rates, suchas a High Rate Packet Data (HRPD) or High Data Rate (HDR) communicationssystem, a variable data rate request scheme may be used to communicateat the maximum data rate that the C/I may support. The HDRcommunications system is typically designed to conform to one or morestandards such as the “cdma2000 High Rate Packet Data Air InterfaceSpecification,” 3GPP2 C.S0024, Version 2, Oct. 27, 2000, promulgated bythe consortium “3^(rd) Generation Partnership Project.”

[0083] Generally, in an HRPD system, an Access Network (AN) is definedas the network equipment providing data connectivity between a cellularnetwork and a packet switched data network (typically the Internet) andthe ATs. An AN in an HRPD system is equivalent to a base station in acellular communication system. An Access Terminal (AT) is defined as adevice providing data connectivity to a user. An AT in an HRPD systemcorresponds to a mobile station in a cellular communication system. AnAT may be connected to a computing device such as a laptop personalcomputer or it may be a self-contained data device such as a PersonalDigital Assistant (PDA). Note that the terms mobile station, remoteterminal, and access terminal are used interchangeably.

[0084] A receiver in an exemplary HDR communications system employing avariable rate data request scheme is shown in FIG. 4. The receiver 400is a subscriber station in communication with a land-based data networkby transmitting data on a reverse link to a base station (not shown).The base station receives the data and routes the data through a basestation controller (BSC) (also not shown) to the land-based network.Conversely, communications to the subscriber station 400 may be routedfrom the land-based network to the base station via the BSC andtransmitted from the base station to the subscriber unit 150 on aforward link. The forward link refers to the transmission from the basestation to the subscriber station and the reverse link refers to thetransmission from the subscriber station to the base station.

[0085] In the exemplary HDR communications system, the forward link datatransmission from the base station to the subscriber station 400 shouldoccur at or near the maximum data rate which may be supported by theforward link. Initially, the subscriber station 400 establishescommunication with the base station using a predetermined accessprocedure. In this connected state, the subscriber station 400 mayreceive data and control messages from the base station, and is able totransmit data and control messages to the base station. The subscriberstation 400 then estimates the C/I of the forward link transmission fromthe base station 400. The C/I of the forward link transmission may beobtained by measuring the pilot signal from the base station. Based onthe C/I estimation, the subscriber station 400 transmits to the basestation a data rate request message as a Data Rate Control (DRC) messageon an assigned DRC channel. The DRC message may contain the requesteddata rate or, alternatively, an indication of the quality of the forwardlink channel, e.g., the C/I measurement itself, the bit-error-rate, orthe packet-error-rate. The base station uses the DRC message from thesubscriber station 400 to efficiently transmit the forward link data atthe highest possible rate.

[0086] The BSC (not shown) may interface with a packet networkinterface, a PSTN, and/or other base stations, and serves to coordinatethe communication between subscriber stations and other users.

[0087] The forward link pilot channel provides a pilot signal, which maybe used by the subscriber station 400 for initial acquisition, phaserecovery, and timing recovery. In addition, the pilot signal may also beused by subscriber station 400 to perform the C/I measurement. In thedescribed exemplary embodiment, each time slot on the forward link is2048 chips long with two pilot bursts occurring at the end of the firstand third quarters of the time slot. Each pilot burst is 96 chips induration. Each slot has two parts, wherein each half slot includes apilot burst.

[0088] The forward link transmission is received by an antenna at thesubscriber station 400. The received signal is routed from the antennato a receiver within analog preprocessing unit 402, matched filter 404,and Analog to Digital (A/D) converter 406. The receiver filters andamplifies the signal, downconverts the signal to baseband, quadraturedemodulates the baseband signal, and digitizes the baseband signal. Thedigitized baseband signal is coupled to a demodulator. The demodulatorincludes carrier and timing recovery circuits and further includes theequalizer 410. The equalizer 410 compensates for ISI and generatessymbol estimates from the digitized baseband signal. The symbolestimates are coupled to a controller 416 via communication bus 420. Thecontroller then generates the DRC message. The output of the equalizer410 is also provided to decoder 412. The decoder 412, the equalizer 410,and the controller 416 are each coupled to communication bus 420.

[0089] In addition to generating the DRC message, the controller 416 maybe used to support data and message transmissions on the reverse link.The controller 416 may be implemented in a microcontroller, amicroprocessor, a digital signal processing (DSP) chip, an ASICprogrammed to perform the function described herein, or any otherimplementation known in the art. A timing unit 414 is also coupled tothe communication bus 420. The exemplary embodiment includes a samplememory storage unit 408 coupled to the equalizer 410 and the controller416 via the communication bus 420.

[0090] A RAKE 418 is also coupled to the communication bus 420 andreceives inputs for processing via a structure such as illustrated inFIG. 1. An equalizer controller 422 receives the estimates from the RAKE418, and from the hybrid equalizer 410 when operating. The equalizercontroller 422 then determines when the equalizer is to be used andinitiates operation. Similarly, equalizer controller 422 determines whenthe equalizer is not to be used and initiates termination of operation.Various monitoring units may be implemented to check operating metrics,such as channel quality and/or channel velocity. The equalizercontroller 422 uses such information to make equalizer decisions.

[0091] Performance Measurement

[0092] As described hereinabove, the equalizer configuration may beselected based on a measurement of the SINR, C/I or other performancecriteria. Other performance criteria may include, for example, the MeanSquare Error of the equalizer configuration measured on the pilotsamples. For example, if the equalizer outputs on pilot samples aregiven by {{circumflex over (γ)}_(n):n=1, . . . , K} and the desiredpilot symbols are denoted by {γ_(n): n=1, . . . , K}, the Mean SquareError (MSE) for this configuration is given by: $\begin{matrix}{{MSE} = {\frac{1}{K}{\sum\limits_{n = 1}^{K}\quad {{{{\hat{y}}_{n} - y_{n}}}^{2}.}}}} & {{Eq}.\quad (7)}\end{matrix}$

[0093] One definition of the SINR or C/I estimate is the following:$\begin{matrix}{{SINR} = {\frac{1}{MSE} - 1.}} & {{Eq}.\quad (8)}\end{matrix}$

[0094] Other definitions or performance measures are also possible.

[0095] The models, methods, and apparatus presented hereinabove serve asexamples of various embodiments supporting different systems, channelconditions, and receiver designs. The application of parallel equalizersas described hereinabove may be implemented in any of a variety ofreceivers adapted for operation in a variety of communication systems,including but not limited to high data rate systems.

[0096] Those skilled in the art will appreciate that the variousillustrative logical blocks, modules, circuits, and algorithms describedin connection with the embodiments disclosed herein may be implementedas electronic hardware, computer software, or combinations of both. Toclearly illustrate this interchangeability of hardware and software,various illustrative components, blocks, modules, circuits, andalgorithms have been described above generally in terms of theirfunctionality. Whether such functionality is implemented as hardware orsoftware depends upon the particular application and design constraintsimposed on the overall system. Skilled artisans may implement thedescribed functionality in varying ways for each particular application,but such implementation decisions should not be interpreted as causing adeparture from the scope of the present invention.

[0097] The various illustrative logical blocks, modules, and circuitsdescribed in connection with the embodiments disclosed herein may beimplemented or performed with a general purpose processor, a digitalsignal processor (DSP), an application specific integrated circuit(ASIC), a field programmable gate array (FPGA) or other programmablelogic device, discrete gate or transistor logic, discrete hardwarecomponents, or any combination thereof designed to perform the functionsdescribed herein. A general purpose processor may be a microprocessor,but in the alternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

[0098] The methods or algorithms described in connection with theembodiments disclosed herein may be embodied directly in hardware, in asoftware module executed by a processor, or in a combination of the two.A software module may reside in RAM memory, flash memory, ROM memory,EPROM memory, EEPROM memory, registers, hard disk, a removable disk, aCD-ROM, or any other form of storage medium known in the art. Anexemplary storage medium is coupled to the processor such the processormay read information from, and write information to, the storage medium.In the alternative, the storage medium may be integral to the processor.The processor and the storage medium may reside in an ASIC. The ASIC mayreside in a user terminal. In the alternative, the processor and thestorage medium may reside as discrete components in a user terminal.

[0099] The previous description of the disclosed embodiments is providedto enable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

What is claimed is:
 1. A method of receiving data in a wirelesscommunication system, the method comprising: processing received signalsthrough a RAKE processing element to generate RAKE processed signals;measuring a first quality metric of the RAKE processed signals;comparing the first quality metric of the RAKE processed signals to afirst threshold value; and when the first quality metric exceeds thefirst threshold value, enabling an equalizer.
 2. The method of claim 1further comprising: measuring a correction metric of the RAKE processedsignals; and comparing the correction metric to a second thresholdvalue, wherein enabling the equalizer further comprises: when the firstquality metric exceeds the first threshold value and the correctionmetric exceeds the second threshold value, enabling an equalizer.
 3. Themethod of claim 2, wherein the first quality metric is a signal to noiseratio.
 4. The method of claim 2, wherein the correction metric is across-correlation measure.
 5. The method of claim 4, wherein thecross-correlation is measured between pilot bursts.
 6. The method ofclaim 2, wherein after enabling the equalizer: the method furthercomprises: measuring the first quality metric of the equalizer processedsignals; comparing the first quality metric of the equalizer processedsignals to the first quality metric of the RAKE processed signals; andwhen the first quality metric of the equalizer processed signals is lessthan the first quality metric of the RAKE processed signals disablingthe equalizer.
 7. A method of receiving data in a wireless communicationsystem, the method comprising: processing received signals through aRAKE processing element to generate RAKE processed signals; andperiodically testing operating conditions, comprising: processingreceived signals through an equalizer to generate equalizer processedsignals; measuring a first quality metric of the RAKE processed signals;measuring the first quality metric of the equalizer processed signals;comparing the first quality metric of the RAKE processed signals to thefirst quality metric of the equalizer processed signals; and determiningwhether to enable the equalizer based on the comparison.
 8. The methodof claim 7, wherein if the first quality metric of the RAKE processedsignals exceeds the first quality metric of the equalizer processedsignals by a margin amount, then determining whether to enable theequalizer based on the comparison comprises determining to disable theequalizer.
 9. The method of claim 8, wherein if the first quality metricof the RAKE processed signals does not exceed the first quality metricof the equalizer processed signals by the margin amount, thendetermining whether to enable the equalizer based on the comparisoncomprises determining to enable the equalizer.
 10. The method of claim9, wherein the first quality metric is a signal to interference andnoise ratio.
 11. The method of claim 10, wherein when the equalizer isenabled, the method further comprises: terminating testing; processingreceived signals through the equalizer to generate equalizer processedsignals; measuring the first quality metric of the RAKE processedsignals; measuring the first quality metric of the equalizer processedsignals; comparing the first quality metric of the RAKE processedsignals to the first quality metric of the equalizer processed signals;and determining whether to disable the equalizer based on thecomparison.
 12. The method of claim 11, wherein if the first qualitymetric of the RAKE processed signals exceeds the first quality metric ofthe equalizer processed signals by a margin amount, then determiningwhether to disable the equalizer based on the comparison comprisesdetermining to disable the equalizer.
 13. The method of claim 12,wherein if the first quality metric of the RAKE processed signals doesnot exceed the first quality metric of the equalizer processed signalsby the margin amount, then determining whether to disable the equalizerbased on the comparison comprises determining to enable the equalizer.14. The method of claim 13, wherein periodically testing operatingconditions, further comprises: initiating the testing once in a sampleperiod, wherein the sample period is a function of a time constant of anequalizer filter.
 15. An apparatus for receiving data in a wirelesscommunication system, the method comprising: means for processingreceived signals through a RAKE processing element to generate RAKEprocessed signals; means for measuring a first quality metric of theRAKE processed signals; means for comparing the first quality metric ofthe RAKE processed signals to a first threshold value; and means forenabling an equalizer when the first quality metric exceeds the firstthreshold value.
 16. A receiver in a wireless communication system, thereceiver comprising: processing element for processing computer-readableinstructions; and memory storage device adapted to storecomputer-readable instructions comprising: a first set ofcomputer-readable instructions for processing received signals through aRAKE processing element to generate RAKE processed signals; a first setof computer-readable instructions for measuring a first quality metricof the RAKE processed signals; a first set of computer-readableinstructions for comparing the first quality metric of the RAKEprocessed signals to a first threshold value; and a first set ofcomputer-readable instructions for enabling an equalizer when the firstquality metric exceeds the first threshold value.
 17. An apparatus forreceiving data in a wireless communication system, the apparatuscomprising: means for processing received signals through a RAKEprocessing element to generate RAKE processed signals; and means forperiodically testing operating conditions, comprising: means forprocessing received signals through an equalizer to generate equalizerprocessed signals; means for measuring a first quality metric of theRAKE processed signals; means for measuring the first quality metric ofthe equalizer processed signals; means for comparing the first qualitymetric of the RAKE processed signals to the first quality metric of theequalizer processed signals; and means for determining whether to enablethe equalizer based on the comparison.
 18. A receiver in a wirelesscommunication system, the receiver comprising: processing element forimplementing computer-readable instructions; and memory storage devicefor storing computer-readable instructions for: processing receivedsignals through a RAKE processing element to generate RAKE processedsignals; and periodically testing operating conditions by: processingreceived signals through an equalizer to generate equalizer processedsignals; measuring a first quality metric of the RAKE processed signals;measuring the first quality metric of the equalizer processed signals;comparing the first quality metric of the RAKE processed signals to thefirst quality metric of the equalizer processed signals; and determiningwhether to enable the equalizer based on the comparison.
 19. A wirelesscommunication apparatus, comprising: a RAKE receiver adapted to receivea signal and generate an estimate of the received signal; an equalizer;and an equalization controller adapted to control operation of theequalizer in response to the estimate from the RAKE receiver.
 20. Theapparatus as in claim 19, wherein the equalization controller enablesthe equalizer when a channel quality measure of the estimate is above athreshold value.
 21. The apparatus as in claim 20, wherein theequalization controller enables the equalizer when the channel qualitymeasure of the estimate is above the threshold and a first correlationof the estimate is greater than a second correlation of an equalizedestimate generated by the equalizer.
 22. The apparatus as in claim 21,wherein the first correlation and the second correlation are based onreceived pilot signals.
 23. The apparatus as in claim 19, wherein theequalization controller disables the equalizer when a channel qualitymeasure of the estimate from the RAKE receiver is greater than a channelquality measure of an equalized estimate generated by the equalizer. 24.The apparatus as in claim 19, wherein the equalization controllerperiodically enables the equalizer to compare an equalized estimategenerated by the equalizer to the estimate from the RAKE receiver. 25.The apparatus as in claim 24, wherein the equalization controllercompares channel quality measures of the equalized estimate generated bythe equalizer and the estimate from the RAKE receiver.
 26. The apparatusas in claim 24, wherein the equalization controller compares channelvelocity of the equalized estimate generated by the equalizer and theestimate from the RAKE reeiver.
 27. The apparatus as in claim 19,wherein the equalizer is adapted to operate in a first operating modeand in a second test mode when enabled.
 28. The apparatus as in claim27, wherein the equalizer transitions from the second test mode to thefirst operating mode when a channel quality measure of an equalizedestimate generated by the equalizer is greater than a channel qualitymeasure of the estimate from the RAKE receiver.
 29. The apparatus as inclaim 28, wherein the equalization controller disables the equalizerwhen a signal-to-noise ratio of the estimate from the RAKE receiver isgreater than an equalized estimate from the equalizer.
 30. The apparatusas in claim 19, wherein the apparatus has two operating modes,comprising: a first mode wherein the RAKE receiver is enabled and theequalizer is disabled; a second mode wherein the RAKE receiver andequalizer are enabled.
 31. The apparatus as in claim 19, wherein theapparatus is adapted for two configurations, comprising: a firstconfiguration wherein the RAKE receiver is enabled and the equalizer isdisabled; a second configuration wherein the RAKE receiver and equalizerare enabled.
 32. The apparatus as in claim 30, wherein the apparatus hasa third operating mode, comprising: a test mode wherein the equalizer isenabled for a sample period and an equalized estimate compared to theestimate from the RAKE receiver.